The present invention is directed to an edge detector system for use in controlling the position of the edge of a flexible web or similar material. In particular, the invention is directed to a ratiometric edge detector system for detecting the position of the edge of a web in a web processing machine and for generating a control signal for use in re-positioning the edge to a pre-selected or home position.
In the manufacture of corrugated paperboard, three plies of paper are combined to form a rigid, continuous web of board. The web may be slit, scored and cut to suitable sizes to form sheets which are subsequently processed into boxes. In combining the plies of paper, the plies must be maintained in relatively close relation to avoid waste. Additionally, the combined board must not be allowed to drift from its preselected path while being slit and scored. Various web-guiding and tracking mechanisms are known for confining the webs of paper and board to a preselected path. For the most part, the mechanisms have been troublesome and unreliable.
In general, the web-guiding and tracking mechanisms known in the art fall into two classes: mechanical and optical. Those devices which mechanically sense the edge of the paper are structurally complex. In addition, the edge of the web may not be sufficiently rigid to exert an adequate force on the mechanical sensing member employed, rendering the mechanism unreliable. The optical mechanisms known in the art may not be able to cope with fluctuations in the texture and color of the web. The mechanisms may not be able to compensate for variations in the distances between the optical components and the web during web travel. Further, the mechanisms often require monitoring of both edges of the web and may be insensitive to lateral movement of the web within a deadband.
U.S. Pat. No. 3,368,726 describes an optical web tracking mechanism which suffers from the afore-mentioned disadvantages. The mechanism includes first and second optical assemblies. The first assembly includes a light source and a pair of binary optical sensors juxtaposed with one edge of the web. The second assembly includes a light source and a single binary optical sensor juxtaposed with the opposite edge of the web. The light sources and sensors must be disposed on both sides of the web.
When the web is in the home position, two of the sensors are blocked from receiving light by the web. Only the third sensor is exposed to light. Lateral movement of the web beyond a deadband exposes one or more of the blocked sensors and blocks the previously exposed sensor. The change in binary outputs of the sensors is detected to indicate lateral movement of the web. Mounting of the optical components is unduly complex since the components must be oriented on both sides of the web. Lateral movement of the web within the deadband does not apparently result in a change in binary output of the optical components. Tracking of the edge position, therefore, is relatively imprecise.
U.S. Pat. No. 3,753,381 also describes an optical web tracking mechanism. The mechanism includes a pair of optical sensors. One sensor detects light reflected from the web while the other sensor detects light reflected from a reference background. Lateral movement of the web beyond a deadband results in either of two conditions. First, both sensors may detect light reflected from the web. Alternatively, both sensors may detect light reflected from the reference background. The optical components are apparently sensitive to variations in web texture and color. Apparently, the mechanism cannot compensate for variations in the distance between the optical components and the web during web travel. The mechanism is particularly objectionable because it requires a reference surface or light source to be mounted below the web. In paper processing operations, dust often accumulates on the surfaces of machine components. The accumulation of dust on the reference surface could result in an erroneous measurement of edge position.
U.S. Pat. No. 3,090,434 also describes an optical web tracking mechanism. The mechanism includes two pairs of light sources and optical sensor disposed along both edges of the web. Lateral movement of the web continuously varies the amount of light received by each sensor. At the home position of the web, the sensors detect equal amounts of light. The mechanism is objectionable because it requires mounting of optical components on both sides of the web, and because the optical components must be mounted in juxtaposition with both edges of the web. The mechanism apparently is unable to compensate for variations in the distance between the optical components and the web during web travel.
An advantage of the present invention is that the position of the edge of the web can be precisely controlled with no deadband.
Another advantage of the invention is that the position of the edge of the web can be controlled independently of texture and color variations of the web.
A further advantage of the invention is that the optical components can be conveniently mounted in proximity on one side of the web so that adjustment of repair of the components can be easily effected.
A still further advantage of the invention is that it automatically compensates for variations in the distance between the optical components and the web during web travel.